This invention relates to an air meter, particularly to an air meter for measuring air mass flow suitable for the use of engine control for an automobile.
As the prior art air meter, a ratio-metric output type air meter is known having an output characteristic adjusting circuit and a ratio-metric converting circuit, as shown in Japanese Patent Prepublication No. 148556/1998. Also, in a frequency output type air meter, a ratio-metric output type air meter is known having an output characteristic adjusting circuit and a voltage controlled oscillator circuit.
However, in both of the ratio-metric output type air meter and the frequency output type air meter, two circuits of the respective air meters have independent internal reference power supply circuits, and to assure their accuracy it is necessary that a resistor is connected to the external of a monolithic IC, and therefore, it is necessary that the IC has connection terminals of the number proportional to the number of the internal reference power supply circuit outputs.
An object of this invention is to provide an air meter in which in case where a plurality of internal reference power supply circuits are needed, it is possible to improve the accuracy of the internal reference voltage with fewer IC terminals.
The above-mentioned and other objects of this invention can be attained by an air meter having a characteristic adjusting circuit for adjusting the characteristic of a flow rate signal detected by flow rate detecting means, by using a first internal reference voltage, and outputting the flow rate characteristic signal, a conversion circuit for converting the flow rate characteristic signal which this characteristic adjusting circuit outputs, to a ratio-metric output or a frequency output on the basis of a second internal reference voltage, and a reference power supply circuit for said first and second internal reference voltages, said internal reference power supply circuit comprising: a temperature current generating circuit for generating a current proportional to a temperature; a current mirror circuit for deprecating the current of this temperature current generating circuit; a first reference voltage generating circuit for flowing the current duplicated by this current mirror circuit and generating said first reference voltage; and a second reference voltage generating circuit for flowing another current duplicated said current mirror circuit and generating said second reference voltage.
With this arrangement, it is possible to improve the precision of the internal reference voltages with few terminal of IC.
In accord with another aspect of this invention, preferably said temperature current generating circuit uses as a resistor a thick-film resistor, and said first and second reference voltage generating circuits use a monolithic resistor and a thick-film resistor, respectively.
With this arrangement, it is possible to correct the linearity of the temperature characteristics of the reference voltages.